It is possible to use a III-V compound semiconductor for a semiconductor device such as a power device other than a light emitting diode (LED) or the like. For such a power device that uses a III-V compound semiconductor, there is provided a high electron mobility transistor (HEMT) that is composed of an electron supply layer and an electron transient layer. In an HEMT, a two-dimensional electron gas (2 DEG) is generated in an electron transient layer near an interface between an electron supply layer and the electron transient layer. Because an operation of such an HEMT is made by transfer of a 2 DEG in an electron transient layer, a high quality crystal film that is formed by an epitaxial growth is desired for the electron transient layer. Herein, a high quality crystal film means a state of no atomic vacancy and no disturbance of an atomic position. For an electron transient layer, a GaN film is frequently used that is a representative III-V compound semiconductor. In a case where a GaN film is used for an electron transient layer, an epitaxial growth is conducted by using a GaN substrate so that it is possible to readily obtain a high quality crystal film.
However, it is difficult to supply an HEMT that uses a GaN or the like at a low cost, because a GaN substrate is very expensive. For this reason, a method for forming a high quality GaN film via a buffer layer on a silicon carbide (SiC) substrate, a sapphire (Al2O3) substrate or even further an inexpensive silicon (Si) substrate has been attempted in recent years. An SiC substrate, an Si substrate, or the like is such that a lattice constant is different between a material for forming a substrate and a material for forming a film, differing from a GaN substrate. For this reason, in a case where lattice mismatch is large, disturbance of an atomic position in a film is large and further a vacancy may be frequently generated.
For a method for evaluating a disturbance of an atomic position in a GaN film or the like, an X-ray diffraction (XRD) method is commonly used. For example, a disturbance of a crystalline orientation in a direction vertical to a substrate surface is determined from a half-value width of a symmetric diffraction peak in a rocking curve measurement (tilt), and further, a disturbance of a crystalline orientation in an in-plane direction is determined from a half-value width of an asymmetric diffraction peak in an in-plane rotation measurement (twist). These values are correlated with a dislocation density, and frequently used as a method for evaluating a disturbance of an atomic position. However, an XRD is a very macroscopic measurement method, and hence, may be difficult to be sufficient in a case where an evaluation of a disturbance at a more atomic level is desired. Furthermore, in an XRD method, it is also impossible to distinguish and evaluate a disturbance of a group III element such as gallium (Ga) and a disturbance of a group V element such as nitrogen (N).
For a method for evaluating a vacancy in a GaN film or the like, a photo-luminescence (PL) method is commonly used. Specifically, an intensity of a yellow luminescence observed near a wavelength of 570 nm is sensitive to an amount of a vacancy of a group III element (Ga) and is commonly used. Moreover, an intensity of a luminescence at a band edge that does not depend on an amount of a vacancy and is observed near 365 nm is frequently used as a reference, and a value is also frequently used provided by normalizing an intensity of a yellow luminescence with an intensity of a luminescence at a band edge.
Furthermore, a positron annihilation method has been known as another vacancy evaluation method, although it is not commonly used more than a PL method. A positron annihilation method is a method that irradiates a sample with a positron and detects a gamma ray generated at a time of annihilation of the positron so that a period of time until the annihilation is measured. A period of time for annihilation of a positron is sensitive to a vacancy and its sensitivity greatly depends on a charge of a vacancy. In a case of GaN, only a vacancy of Ga that is a group III element is reflected. The aforementioned two vacancy evaluation methods, that is, both a PL method and a positron annihilation method are sensitive to a vacancy of a group III element, and an evaluation method for a vacancy of N that is a group V element, that is, GaN, has not been established.
Japanese Laid-Open Patent Application No. 2009-147271
Japanese Laid-Open Patent Application No. 2012-089651
Japanese Laid-Open Patent Application No. 2011-027528
Japanese Laid-Open Patent Application No. 2002-280433